The short answer
A real certificate of analysis names the laboratory, carries a batch or lot number that matches your bottle, lists the test methods, and gives numeric results for identity, potency, heavy metals, and microbiology. If any one of those is missing, you are looking at a document designed to be printed rather than read.
Third-party tested has no regulatory definition for dietary supplements. Unlike drugs, supplements do not need premarket approval, and a company can print the phrase without ever sending a sample to an outside lab. The certificate is where the claim either holds up or does not.
One: the laboratory is named
A COA should say who ran the tests. ISO 17025 accredited laboratories are the ones to look for, because accreditation means the methods and the equipment are audited. Common names in this industry include Eurofins, NSF, USP, ConsumerLab, and a long list of regional contract labs.
An in-house COA from the manufacturer is not worthless, and plenty of reputable companies run their own quality labs. It is not third party, though, and it should not be described as one.
Two: the batch number matches your bottle
This is the test that separates a real program from a scanned document. A COA applies to a single lot. If the certificate on a website has no lot number, or the number does not match the lot printed on your bottle, it tells you nothing about what is in your bottle.
Some brands publish a batch lookup page where you enter the lot code and get the matching certificate. That is the version worth using, because it ties the document to the physical product instead of to a marketing page.
Three: identity testing
Identity testing confirms the botanical or compound in the bottle is the one on the label. For botanicals this is usually done by chromatography or by DNA barcoding, and the reason it matters is that herbal raw materials get substituted and adulterated, sometimes deliberately and sometimes through a supply chain error nobody caught.
This is the least glamorous line on the certificate and it is the one most likely to be skipped. If you are buying a botanical, ask whether identity was confirmed and how.
Four: potency, as a number
Potency testing measures how much of the active compound is actually present, and the useful version gives you a number, not a pass. "Meets specification" is a pass. "Monacolin K 2.4 mg per serving" is a result.
Potency is also where the shelf life discussion comes back. A product overfilled at manufacture to survive degradation will test high early and lower later, which is one reason a certificate tied to your actual lot beats a generic one from two years ago.
Five: contaminants, with limits
Four groups show up on a serious certificate.
Heavy metals, usually lead, arsenic, cadmium, and mercury, reported in parts per million or micrograms per serving. California's Proposition 65 sets a maximum allowable dose level for lead at 0.5 micrograms per day for products that require a warning, and while Prop 65 is a warning statute rather than a safety standard, it is the number most supplement companies in the US formulate against.
Microbial limits, reported as total aerobic plate count, yeast and mold, and absence of specific pathogens like E. coli, Salmonella, and Staphylococcus aureus. Botanicals and any water containing format need this more than dry capsules do.
Residual solvents, which matter for extracts made with ethanol or hexane, and which should be reported as not detected or below a stated limit.
Pesticide residues for agricultural raw materials, which is the panel most likely to be missing from a cheap certificate.
What the seals mean
The three marks you will see in the US are USP Verified, NSF Certified, and ConsumerLab Approved Quality. They are not interchangeable.
USP Verified means the product was tested for identity, potency, and contaminants, and that the manufacturer's facility was audited. NSF certification does similar testing plus label claim verification and facility inspection. ConsumerLab is a subscription service that buys products off the shelf and tests them independently, which is a different and in some ways more honest model, because the manufacturer does not choose what gets tested.
Any of these is a meaningful signal. A company logo that merely looks like a seal is not.
Why this matters more for some categories
Botanicals and glandular products have the most room for substitution and contamination. Minerals from mined sources carry more heavy metal risk than synthetically produced vitamins. Anything grown outdoors and dried, which is most herbs, needs the microbial and pesticide panels.
Single ingredient synthetic vitamins have the least to hide, which is why a cheap ascorbic acid bottle and a cheap herbal extract are not comparable purchases even when the price is similar.
How to ask
Email the company and ask for the COA for the lot you own. A company with a real testing program will send it, usually the same week. A company that sends a generic PDF with no lot number, or a page of marketing copy about their commitment to quality, has told you what you needed to know.
You can start with our zinc carnosine capsules or any bottle in the Velnazivs line and ask for the lot you have.
Is third-party tested a regulated claim
No. There is no FDA definition for it in the supplement context, which is why the phrase appears on bottles with no outside testing behind it. The certificate, not the phrase, is the evidence.
What is ISO 17025 accreditation
It is the international standard for testing and calibration laboratories. Accredited labs are audited on their methods, equipment, and competence. It is the baseline credential to look for in the laboratory name at the top of a COA.
Does a COA prove the supplement works
No. It proves what is in the bottle and what is not. Efficacy is a separate question, and a clean certificate of analysis for a compound with no human evidence still leaves you with a compound that has no human evidence.
Should I avoid products without one
For botanicals and glandulars, yes, or at least treat the absence as a meaningful discount on the price you are willing to pay. For simple synthetic vitamins, the risk is lower and the certificate matters less.